By using a reflecting/refractive system and hardware devices such as fish-eye lens, seamless wide-view images can be established directly. However, such hardware devices capture as much information as possible on a limited imaging plane, as a result, the seamless wide-view images are distorted seriously.
A digital imaging device photographs images in a digital format. Multiple digital images are stitched together to form a wide-view digital panoramic image. The final digital panoramic image involves little distortion. The prevalent image stitching technologies are as follows:
(1) Multiple images are obtained by photographing from different view directions at a fixed viewpoint, and then the point-to-point mapping relation (also known as homography) between the images is calculated, and the relation is primarily reflected as pan, tilt, affine and projection transformation; afterward, all images are projected onto a uniform coordinate system according to the mapping relation, and stitched into a wide-view image or into a 360-degree image; finally, the seams on the overlap region formed in the stitching process of images are eliminated by using a smooth function.
This image stitching method is generally applied to virtual roaming on the Internet. People who visit a website can browse the panoramic images of objects like those in the real world. However, in this stitching method, only the images obtained by photographing at a fixed viewpoint make sense. All images have to be obtained by photographing at the same static position. Otherwise, the stitched image has ghosts due to parallax, and the stitching region is unnatural. In practice, it is hard to obtain images by photographing at the same static position, namely, it is hard to rotate the camera around the optical center to make optical centers of all images coincide.
(2) After the images are obtained from different viewpoints, certain continuous intermediate points among the viewpoints are used as new viewpoints to reconstruct images, and then the overlap region between the old image and the reconstructed image is found, and the images are stitched according to the overlap region.
The method for using intermediate points as viewpoints to reconstruct images can deghost the stitched image which has ghosts caused by parallax between two viewpoints. However, if the parallax of the original images is too large, too many intermediate point images need to be generated to deghost the stitched image, and enormous calculations are required. When images of more than two viewpoints are stitched, too many intermediate point images are generated, and the image stitching takes too much time.
(3) The focal length of the camera, principal point coordinates, distortion factor, rotation angle, and rotation position are obtained through calibration of the camera, in order to calculate the mapping relation for the images obtained by the camera.
The third method above involves calibration of the camera, which is difficult in the computer field. Meanwhile, in this method, the backgrounds of the obtained images need to be set as a single background to ignore parallax of backgrounds in parallax compensation.